On the long period character of shear waves

1961 ◽  
Vol 51 (1) ◽  
pp. 1-12
Author(s):  
Jack Oliver
Keyword(s):  
Surface Wave ◽  
Particle Motion ◽  
Shear Waves ◽  
Wave Guide ◽  
Near Surface ◽  
Long Period ◽  
Data Support ◽  
Surface Particle ◽  
Wave Trains ◽  
Period Wave

Abstract S and multiple S phases at moderate to large epicentral distances are frequently followed by normally-dispersed, long-period, wave trains for which surface particle motion is elliptical and progressive and in the plane of propagation of the SV wave. The character of such phases can be explained as the result of coupling between the incident shear waves and dispersive PL waves in the near-surface wave guide. A detailed study of shocks in Mexico and in Montana recorded at Resolute, and less detailed studies of other data support this hypothesis.

Near-surface scattering effects observed with a high-frequency phased array at Pinyon Flats, California

1998 ◽  
Vol 88 (6) ◽  
pp. 1548-1560
Author(s):  
Frank L. Vernon ◽  
Gary L. Pavlis ◽  
Tom J. Owens ◽  
Dan E. McNamara ◽  
Paul N. Anderson
Keyword(s):  
Surface Wave ◽  
Wave Field ◽  
Particle Motion ◽  
High Frequency ◽  
Body Wave ◽  
Three Dimensional ◽  
Wave Guide ◽  
Surface Scattering ◽  
Near Surface ◽  
Frequency Wave

Abstract Analysis of data collected by a high-frequency array experiment conducted at Pinyon Flat in southern California provides strong evidence that the high-frequency wave field from local earthquakes at this hard-rock site are strongly distorted by near-surface scattering. The seismic array we deployed consisted of 60, 2-Hz natural frequency, three-component sensors deployed in a three-dimensional array. Two of the sensors were located in boreholes at 150 and 275 m depth. The other 58 sensors were deployed in an areal array above these boreholes. Thirty-six of these were deployed in a 6-by-6 element grid array with a nominal spacing of 7 m centered over the borehole sensors. The remaining 22 seismometers were laid out in two 11-element linear arrays radiating outward from the grid. Coherence calculations reveal a rapid loss of coherence at frequencies over 15 Hz at all but the shortest length scales of this array. Three-dimensional visualization techniques were used to closely examine the spatial stability of particle motions of P and S waves. This reveals systematic variations of particle motion across the array in which the particle motion tracks tilt drastically away from the backazimuth expected for an isotropic medium. These variations, however, are frequency dependent. Below around 8 Hz, the particle motions become virtually identical for all stations. At progressively higher frequencies, the wave-field particle motion becomes increasingly chaotic. Frequency-wave-number analysis of these data provide quantitative measures of the same phenomena. We find that direct wave f-k spectra are bathed in a background of signal-generated noise that varies from 10 to 30 dB down from the direct arrival signal. This signal-generated noise appears to be nearly white in wavenumber indicating the wavelength of this “noise” on the scale of tens of meters and less. Refraction measurements we made on two lines crisscrossing the array reveal that the weathered layer velocities are highly variable and define a very strong wave guide. Measured surface P-wave velocities varied from 400 to 1300 m/sec, and velocities at depth of approximately 15 m varied from 1600 to 2700 m/sec. Previous measurements in the boreholes showed that the intact granite below about 65 m depth has a velocity of approximately 5400 m/sec. These results demonstrate the extreme velocity contrast and degree of velocity heterogeneity of the near surface at this site. We conclude that all the observations we made can be explained by strong scattering of incident body-wave signals into a complex mishmash of body-wave and surface-wave modes in this heterogeneous near-surface wave guide.

Long period seismic waves from large, near-surface nuclear explosions

1963 ◽  
Vol 53 (1) ◽  
pp. 109-149 ◽  
Author(s):  
Paul W. Pomeroy
Keyword(s):  
Hong Kong ◽  
Surface Wave ◽  
Initial Phase ◽  
Love Waves ◽  
Marshall Islands ◽  
P Wave ◽  
Test Series ◽  
Near Surface ◽  
Long Period ◽  
The U.S

Abstract Seismic surface waves were well-recorded from the larger explosions of the U.S. test series detonated in the Marshall Islands during the spring and summer of 1958 and of the U. S. S. R. test series detonated at the Novaya Zemlaya test site during October 1958. In addition to waves of the fundamental Rayleigh mode, some unusual or unexpected waves were identified. These include: 1.) Love waves with lengths as great as 90 km at Hong Kong from the U. S. S. R. explosions, 2.) Love waves at Agra, India, from the U. S. S. R. explosions, 3.) waves of the 1st shear mode at Agra and Uppsala, Sweden, from the U. S. S. R. explosions, and 4.) Love, 1st shear and PL waves at Guam from the U. S. explosions. Group velocity data were derived for many paths and, for the U. S. series, these data are very accurate because they are based on exact knowledge of locations and origin times. For the U. S. S. R. explosions, revised locations and origin times based on a limited number of P-wave observations were used to determine group velocities. Using records from Hong Kong and Honolulu for two U. S. tests, one at Eniwetok and one at Bikini, oceanic phase velocities in the period range of 15 to 40 seconds were measured for the path between these islands. Using Brune's method for initial phase determination with a known phase velocity, an initial phase, φ0, lying between +3π2 and +π2 was determined for the U.S. explosion, Oak. According to Aki (1960), these values of initial phase are associated, respectively, with forcing functions of a downward impulse and an explosive impulse. Seismic magnitudes of 4.7 and 4.8 were assigned to the U. S. Oak and Poplar events on the basis of the surface wave data, while magnitudes of the larger U. S. S. R. tests range from about 4.0 to 4.5. The ratio of seismic energy as computed from the surface wave magnitude to the total explosive energy available (yield) is apparently greater for the U. S. explosions, assuming equal yield for the two shots compared. This suggests a higher altitude of detonation for the Russian events. The Fourier amplitude spectra of the surface wave trains are generally single peaked where a significant portion of the path traversed is oceanic, but for Uppsala, where the path is short and continental, a second peak occurs at periods of about 10 to 13 seconds. The ratios of the predicted amplitudes, based on the data of one station and on a reasonable dissipation factor, to the actual recorded amplitudes vary with azimuth by as much as a factor of 5. These variations may be explained by asymmetry at the source but might also be explained by instrumental and geologic factors. Long period components of P and S and multiples thereof were sometimes recorded from these events.

Leaking modes and the PL phase

1960 ◽  
Vol 50 (2) ◽  
pp. 165-180
Author(s):  
Jack Oliver ◽  
Maurice Major
Keyword(s):  
Rayleigh Wave ◽  
Rayleigh Waves ◽  
Wave Train ◽  
Wave Dispersion ◽  
P Wave ◽  
Wave Guide ◽  
Mode Propagation ◽  
Near Surface ◽  
Surface Particle ◽  
Rayleigh Wave Dispersion

ABSTRACT The PL phase is a normally dispersed train of waves of periods greater than about 10 seconds beginning at or near the time of the initial P wave and sometimes continuing at least to the time of the beginning of the Rayleigh-wave train. With adequate instrumentation the PL phase is commonly observed at distances less than about 25° from shallow shocks. In general, surface particle motion is elliptical and progressive, and amplitudes are not greater than about one-quarter those of Rayleigh waves of the same period. Comparison of PL- and Rayleigh-wave dispersion shows that both waves propagate in roughly the same near-surface wave guide. Whereas Rayleigh waves correspond to normal- (nonleaking-) mode propagation, PL waves appear to correspond to leakingmode propagation within this wave guide.

A surface wave particle motion discrimination process

1968 ◽  
Vol 58 (2) ◽  
pp. 629-637
Author(s):  
R. S. Simons
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Particle Motion ◽  
Seismic Data ◽  
Digital Computer ◽  
Rayleigh Waves ◽  
Motion Pattern ◽  
Signal To Noise ◽  
Long Period ◽  
Period Data

Abstract A digital computer process is described which examines the three-componen particle-motion pattern of seismic data as a function of frequency and uses this information to discriminate against all motion except surface waves arriving from some pre-assigned direction. Application of the process to long-period data has shown that it can substantially improve the signal-to-noise ratios of Love and Rayleigh waves from earthquakes and underground detonations.

High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations

1991 ◽  
Vol 81 (4) ◽  
pp. 1057-1080 ◽  
Author(s):  
Richard C. Aster ◽  
Peter M. Shearer
Keyword(s):  
Shear Wave ◽  
Fault Zone ◽  
Particle Motion ◽  
Southern California ◽  
Shear Waves ◽  
P Wave ◽  
S Wave ◽  
Near Surface ◽  
San Jacinto Fault ◽  
San Jacinto Fault Zone

Abstract Two borehole seismometer arrays (KNW-BH and PFO-BH) have been established in the Southern California Batholith region of the San Jacinto Fault zone by the U.S. Geological Survey. The sites are within 0.4 km of Anza network surface stations and have three-component seismometers deployed at 300 m depth, at 150 m depth, and at the surface. Downhole horizontal seismometers can be oriented to an accuracy of about 5° using regional and near-regional initial P-wave particle motions. Shear waves recorded downhole at the KNW-BH indicate that the strong alignment of initial S-wave particle motions previously observed at the (surface) KNW Anza site (KNW-AZ) is not generated in the near-surface weathered layer. The KNW-BH surface instrument, which sits atop a highly weathered zone, displays a significantly different (≈ 20°) initial S-wave polarization direction from that observed downhole and at KNW-AZ, which is bolted to an outcrop. Although downhole initial shear-wave particle motion directions are consistent with a shear-wave splitting hypothesis, observations of orthogonally polarized slow shear waves are generally elusive, even in seismograms recorded at 300 m. A cross-correlation measure of the apparent relative velocities of Sfast and Sslow horizontally polarized S waves suggests shallow shear-wave anisotropy, consistent with the observed initial S-wave particle motion direction, of 2.3 ± 1.7 per cent between 300 and 150 m and 7.5 ± 3.5 per cent between 150 and 0 m.

scholarly journals Computational Modeling of Vortex Generators for Turbomachinery

2002 ◽  
Author(s):  
R. V. Chima
Keyword(s):  
Computational Models ◽  
Body Force ◽  
Secondary Flows ◽  
The Body ◽  
Vortex Generators ◽  
Force Model ◽  
Suction Surface ◽  
Near Surface ◽  
Compressor Rotor ◽  
Surface Particle

In this work computational models were developed and used to investigate applications of vortex generators (VGs) to turbomachinery. The work was aimed at increasing the efficiency of compressor components designed for the NASA Ultra Efficient Engine Technology (UEET) program. Initial calculations were used to investigate the physical behavior of VGs. A parametric study of the effects of VG height was done using 3-D calculations of isolated VGs. A body force model was developed to simulate the effects of VGs without requiring complicated grids. The model was calibrated using 2-D calculations of the VG vanes and was validated using the 3-D results. Then three applications of VGs to a compressor rotor and stator were investigated: 1. The results of the 3-D calculations were used to simulate the use of small casing VGs used to generate rotor preswirl or counterswirl. Computed performance maps were used to evaluate the effects of VGs. 2. The body force model was used to simulate large partspan splitters on the casing ahead of the stator. Computed loss buckets showed the effects of the VGs. 3. The body force model was also used to investigate the use of tiny VGs on the stator suction surface for controlling secondary flows. Near-surface particle traces and exit loss profiles were used to evaluate the effects of the VGs.

scholarly journals Wave Force Characteristics of Large-Sized Offshore Wind Support Structures to Sea Levels and Wave Conditions

2019 ◽  
Vol 9 (9) ◽  
pp. 1855
Author(s):  
Youn-Ju Jeong ◽  
Min-Su Park ◽  
Jeongsoo Kim ◽  
Sung-Hoon Song
Keyword(s):  
Shallow Water ◽  
Wave Height ◽  
Wave Force ◽  
Diffraction Effect ◽  
Support Structures ◽  
Sea Levels ◽  
Irregular Wave ◽  
Long Period ◽  
Short Period ◽  
Period Wave

This paper presents the results of wave force tests conducted on three types of offshore support structures considering eight waves and three sea levels to investigate the corresponding wave forces. As a result of this study, it is found that the occurrence of shoaling in shallow water induces a significant increase of the wave force. Most of the test models at the shallow water undergo a nonlinear increase of the wave force with higher wave height increasing. In addition, the larger the diameter of the support structure within the range of this study, the larger the diffraction effect is, and the increase in wave force due to shoaling is suppressed. Under an irregular wave at the shallow water, the wave force to the long-period wave tends to be slightly higher than that of the short period wave since the higher wave height component included in the irregular wave has an influence on the shoaling. In addition, it is found that the influence of shoaling under irregular wave becomes more apparent in the long period.

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